Most sintered silicon carbides are extremely hard and exhibit good lubricity when used with water lubrication, while their wear resistance and chemical resistance are also excellent. They are therefore widely used in sliding parts such as mechanical seals, bearings and the like. For mechanical seal applications, in particular, they may be used even in situations with a high PV value, which represents the product of the contact pressure and rotation rate on the sliding surface, and therefore sintered silicon carbides (silicon carbide ceramics) are considered to be highly useful materials. Moreover, it is well known that sintered silicon carbides can be used with even higher PV values if pores are introduced therein.
Conventional sintered silicon carbides have exhibited satisfactory properties against sliding friction when liquid films are formed on the sliding surfaces (this will hereinafter be referred to as “lubricated state”). However in the complete absence of liquid films on sliding surfaces or with inadequate formation of liquid films, (this will hereinafter be referred to as “non-lubricated state”), they create solid friction and have therefore tended to cause seizing of sliding surfaces and abnormal wear as a result, when the frictional coefficient is notably increased. Consequently, pure sintered silicon carbides have not necessarily been satisfactory in terms of sliding properties and are less than optimal for use in a non-lubricated state.
As silicon carbide-based sintered bodies with low surface friction, there have been hitherto proposed materials produced by adding carbon materials such as graphite as solid lubricants to silicon carbide. For example, a sintered body obtained by adding carbon black at 1-5 vol % to silicon carbide powder has been disclosed (Patent document 1). The sintered body is described as having a low frictional coefficient with water lubrication. A sintered body obtained by adding carbon black at 6.0-12.0 vol % to silicon carbide powder has also been disclosed (Patent document 2). This sintered body exhibits a satisfactory frictional coefficient in oil tanks.
Composite materials obtained by adding graphite to silicon carbide powder have also been proposed. For example, Patent document 3 discloses a sintered body containing 10-30 wt % flake natural graphite with a mean particle size of 8-100 μm, and having a relative density of 80-92%.
There have also been disclosed ceramic composite materials with a 99.9 wt %-70 wt % silicon carbide content and a 0.1 wt %-30 wt % carbon content, wherein the silicon carbide has a mean particle size of greater than 10 μm and a microstructure with two particle structure modes, while the carbon has a mean particle size of less than 10 μm (see Patent document 4).
[Patent document 1] Japanese Examined Patent Publication SHO No. 61-043310
[Patent document 2] Japanese Unexamined Patent Publication SHO No. 61-063569
[Patent document 3] Japanese Patent Publication No. 3350394
[Patent document 4] Japanese Unexamined Patent Publication No. 2002-326873